Pressure vessels are widely used to store liquids and gases under pressure. The storage capacity of a pressure vessel depends on the internal volume of the pressure vessel and the pressure the vessel is capable of safely containing. In addition to its storage capacity, the size, internal shape, external shape, and weight of the pressure vessel are often important in a particular application.
One growing application of pressure vessels is the storage of compressed natural gas ("CNG"). CNG is increasingly viewed as preferable to gasoline for fueling vehicles. CNG generally burns cleaner than gasoline, leading to a visible reduction in air pollution and corresponding reductions in health care costs. Natural gas is also a relatively abundant fuel. Accordingly, approaches have been devised for converting gasoline-fueled vehicles by retrofitting them to use CNG instead of gasoline.
Known approaches to retrofitting a vehicle for use with CNG include replacing the gasoline tank with conventional natural gas storage cylinders. Unfortunately, the use of conventional CNG cylinders restricts the driving range of the converted vehicle to about 120 to 140 miles, which severely limits consumer acceptance of such conversions. The driving range of such a converted vehicle could be increased by simply adding more CNG storage cylinders. This could be done, for example, by mounting the additional CNG cylinders within the trunk of the vehicle. However, it is generally desirable to fit the CNG storage cylinders within the limited space previously occupied by the gasoline tank.
One suggested approach for increasing the vehicle's driving range is to carry more CNG within the same storage cylinders. This is accomplished by pumping more CNG into the storage cylinders, thereby increasing the pressure within the storage cylinders. However, increasing the storage pressure often requires thickening the walls of the storage cylinders to provide them with sufficient structural strength to resist the higher pressure. Increasing the wall thickness requires either an increase in the external size of the storage cylinders, thereby preventing storage of the cylinders in the space previously occupied by the gasoline tank, or a reduction of the internal storage volume of the cylinders, thereby reducing the volume of stored CNG and hence reducing the vehicle's driving range. Thickening the walls also increases the weight of the storage cylinders, thereby decreasing the fuel efficiency of the vehicle.
Other approaches to increasing the driving range of vehicles fueled by CNG propose varying the shape of CNG storage containers. Currently, spheres, cylinders, and certain combinations of spherical and cylindrical sections are favored. As illustrated in FIGS. 1 and 2, one conventional pressure vessel 100 includes several lobes 102 secured together. Each lobe 102 is geometrically defined as a portion of a "tube-and-dome" shape. Geometrically, a tube-and-dome includes a straight tube 104 which is circular with radius R in transverse cross-section (see FIG. 2). Two lobes 102 are combined by slicing each lobe 102 along a plane 106 that is parallel to the longitudinal axis 108 of the tube 104. The truncated faces of the two lobes 102 are then secured against one another. Each of one or more center lobes 110 is thus sliced along two planes 106 parallel to the longitudinal axis 112 of the center lobe's tube. In the resulting container 100, the lobes 102 are not tangent to one another at junctions 114 where they meet. Each tube 104 is capped at each end by a portion of a hemispherical dome 116 having the same radius R as the tube 104.
Such tube-and-dome containers have several drawbacks when employed in applications requiring substantially rectangular pressure vessels. Such applications include, but are not limited to, storage of CNG for use in fueling a vehicle. The vehicle may be a vehicle retrofitted with CNG tanks after previously being fueled by gasoline, or it may be a vehicle designed from the start to run on CNG.
The drawbacks of tube-and-dome geometry arise from differences between that geometry and a substantially rectangular geometry. In the case of retrofitted vehicles, the desire for substantially rectangular vessels arises because many gasoline tanks are shaped like substantially rectangular shells, as illustrated generally by a phantom rectangular shell 118 in FIGS. 1 and 2. In the case of vehicles designed initially to use CNG, the preference for a substantially rectangular pressure vessel may arise from other design considerations. In either case, a single tube-and-dome lobe 102 is a very poor approximation to such rectangular volumes.
Arranging truncated portions of several tube-and-dome lobes 102 together to form the pressure vessel 100 improves the approximation, but large wedge-shaped unused volumes 120 nonetheless remain which are not used for CNG storage. The unused volumes 120, which are defined by the circular walls of adjacent tube-and-dome lobes 102, may occupy a significant percentage of the internal volume of the rectangular shell 118. Eliminating the unused volumes 120 entirely would require a CNG container which is substantially a rectangular shell in shape. But building a rectangular shell-shaped CNG vessel sufficiently strong to resist typical CNG storage pressures would require excessively thick walls, because the rectangular shell is so far removed in shape from a sphere.
In addition to the unused volumes 120, the vessel 100 has the disadvantage that the lobes 102 tend to peel apart at the junctions 114 because of stresses that occur at the junctions 114. Thickening the walls of the lobes 102 to overcome the peeling tendency reduces the storage capacity of the container 100 or increases its size, and also increases the container's weight.
Thus, it would be an advancement in the art to provide a pressure vessel which approximates a rectangular volume.
It would also be an advancement to provide such a pressure vessel which facilitates the retrofitting of gasoline vehicles by having an external shape compatible with the rectangular shell shape of the exterior of the gasoline tank.
It would be a further advancement to provide such a pressure vessel which has generally circular cross-sections.
It would also be an advancement to provide such a pressure vessel that resists the tendency to peel apart when subjected to internal storage pressures.
Such a pressure vessel is disclosed and claimed herein.